Sequential circuits have memory elements because their present or current output depends on the present inputs and the past history of inputs. What I mean is, the o/p of the Sequential ckt may depend on the past inputs and so it should have some kind of memory to store the state of past i/p's.
Combinational circuits have no memory elements and their present o/p depends on present i/p's only. If you change the i/p's the ckt keeps no knowledge of previous i/p.
A sequential ckt is basically a combination of combinational ckt and memory.